Many studies have heretofore been carried out with regard to aluminum alloys of high strength with starting materials of alloys containing amorphous phases or quasi-crystal phases.
According to the technique disclosed in Japanese Patent Laying-Open No. 1-275732, for example, an amorphous substance or a complex of amorphous and microcrystalline substances having tensile strength of 87 to 103 kg/mm.sup.2 and yield strength of 82 to 96 kg/mm.sup.2 is obtained by rapidly solidifying a ternary alloy consisting of a general formula: Al.sub.a M.sub.b X.sub.c (where M: at least one or two metallic elements selected from V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Ti, Mo, W, Ca, Li, Mg and Si, X: at least one or two metallic elements selected from Y, La, Ce, Sm, Nd, Hf, Nb, Ta and Mm (misch metal), a: 50 to 95 at. %, b: 0.5 to 35 at. % and c: 0.5 to 25 at. %.
An amorphous or microcrystalline high-strength aluminum alloy of low specific gravity and high strength is disclosed in Japanese Patent Laying-Open No. 6-316738. The aluminum alloy is expressed in a general formula: Al.sub.a X.sub.b Mm.sub.c (Mm: misch metal), where X is at least one or two elements selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zr, a, b and c are atomic %, a: 95.2 to 97.5 at. %, and b and c are values satisfying 2.5&lt;b+c&lt;5 and b&gt;0.5 and c&gt;1. Due to having such a composition, there is obtained an aluminum alloy of low specific gravity and high strength in which an amorphous phase or a microcrystal phase is properly homogeneously dispersed in a microcrystal phase of a matrix while suppressing the amount of addition of alloy elements and the microcrystal phase of the matrix is solution-strengthened with Mm and the transition metal such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu or Zr.
As hereinabove described, an amorphous alloy or an alloy consisting of a complex of amorphous and microcrystalline substances, or a microcrystalline alloy having a matrix of Al has tensile strength at least twice that of a conventional aluminum crystalline alloy. However, the Charpy impact value of the aforementioned aluminum alloy is so low that it does not even reach about 1/5 of that of a conventional aluminum ingot material. Thus, there has been such a problem that it is difficult to use the aluminum alloy as the material for a mechanical part or an automobile part which requires reliability.
Japanese Patent Laying-Open No. 6-184712, on the other hand, discloses a method of preparing a high-strength aluminum alloy. The aluminum alloy is expressed in a general formula: Al.sub.a Ln.sub.b M.sub.c, where Ln in the formula is at least one metallic element selected from Mm (misch metal), Y, La, Ce, Sm, Nd, Hf, Nb and Ta, M is at least one metallic element selected from V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Ti, Mo, W, Ca, Li, Mg and Si, a: 50 to 97.5 at. %, b: 0.5 to 30 at. % and c: 0.5 to 30 at. %. The above mentioned Laying-Open Publication also discloses a preparation method that involves performing plastic working on a rapidly solidified aluminum alloy having such a composition and such a cellular diploid structure whereby an amorphous phase of 5 to 50 volume % encloses a microcrystal phase at a temperature exceeding the amorphous crystallization temperature, and obtaining such a structure in which an intermetallic compound consisting of at least two of the aforementioned Al, Ln and M is dispersed in a microcrystal matrix. In such an aluminum alloy, relatively high toughness is obtained such that the tensile strength is 760 to 890 MPa and elongation is 6.0 to 9.0%.
In the preparation method of the aluminum alloy disclosed in the aforementioned gazette, however, it requires a high cooling rate at the time of rapid solidification for obtaining the amorphous phase of 5 to 50 volume %, and hence there is such a problem that the preparation cost increases in actual industrial production.
In Japanese Patent Laying-Open No. 7-179974, further, an aluminum alloy comprising high strength and high toughness is disclosed. The dispersion-strengthened aluminum alloy has a complex structure including a matrix of .alpha.-aluminum and a precipitation phase of an intermetallic compound with a volume ratio of not more than 35 volume % of the intermetallic compound. The aluminum alloy is particularly characterized in that the aspect ratio of the precipitation phase of the intermetallic compound is not more than 3.0, the ratio of the crystal grain size of the .alpha.-aluminum to the grain size of the precipitation phase of the intermetallic compound is at least 2.0, and the crystal grain size of the .alpha.-aluminum is not more than 200 nm. In the aforementioned gazette, further, it is disclosed that the aluminum alloy having the aforementioned limited structure is obtained by performing a first heating treatment and a second heating treatment on gas-atomized powder containing an amorphous phase by at least 10 volume % or a green compact thereof and thereafter performing hot plastic working.
Also in the preparation method of the aluminum alloy disclosed in the aforementioned Laying-Open Publication, it still requires a high cooling rate at the time of rapid solidification for obtaining the amorphous phase of 10 volume %, and hence there is such a problem that the preparation cost therefor increases in actual industrial production.
The problems of the aforementioned conventional techniques are summarized in the following Table 1.
TABLE 1 ______________________________________ Alloy Structure Problem ______________________________________ Japanese Patent amorphous substance or complex of low Laying-Open No. amorphous and microcrystalline toughness 1-275732 substances Japanese Patent microcrystal or microcrystal with low Laying-Open No. amorphous substance dispersed therein toughness 6-316738 Japanese Patent microcrystal with intermetallic requirement Laying-Open No. compound dispersed therein for high 6-184712 quenching degree Japanese Patent microcrystal with intermetallic requirement Laying-Open No. compound dispersed therein for high 7-179974 quenching degree ______________________________________